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Geodynamics Kinematics of plate tectonics Lecture 2.2 - Convergent plate boundaries Lecturer: David Whipp [email protected] Geodynamics www.helsinki.fi/yliopisto 1 Goal of this lecture • Present convergent plate margins 2 Plate boundaries • Three different principal types of plate boundaries • • • Divergent (accreting) Convergent (subduction) Transform 3 Plate boundaries - Convergent (subduction) Fig. 1.7, Turcotte and Schubert, 2014 • Plate boundary where the sense of motion is convergent 4 Plate boundaries - Convergent (subduction) Fig. 1.7, Turcotte and Schubert, 2014 • Plate boundary where the sense of motion is convergent • Subduction destroys oceanic lithosphere, recycling it within the Earth 4 Plate boundaries - Convergent (subduction) Fig. 1.7, Turcotte and Schubert, 2014 • Plate boundary where the sense of motion is convergent • Subduction destroys oceanic lithosphere, recycling it within the Earth • Geometry varies, but most have a trench, accretionary sedimentary prism, volcanic line (or arc) 4 Plate boundaries - Convergent (subduction) Fig. 1.7, Turcotte and Schubert, 2014 • Why does the plate subduct? 5 Plate boundaries - Convergent (subduction) Fig. 1.7, Turcotte and Schubert, 2014 • • Why does the plate subduct? As the lithosphere cools and contracts, its density increases, eventually exceeding that of the underlying asthenosphere 6 Plate boundaries - Convergent (subduction) Fig. 1.7, Turcotte and Schubert, 2014 • • Why does the plate subduct? • This results in a gravitational instability that causes the oceanic plate to sink into the underlying asthenosphere As the lithosphere cools and contracts, its density increases, eventually exceeding that of the underlying asthenosphere 6 Plate boundaries - Convergent (subduction) Fig. 1.7, Turcotte and Schubert, 2014 • • Why does the plate subduct? As the lithosphere cools and contracts, its density increases, eventually exceeding that of the underlying asthenosphere FSP • This results in a gravitational instability that causes the oceanic plate to sink into the underlying asthenosphere • As it sinks, plate motion is driven in part by the dense sinking end of the plate 6 pulling it along via the slab pull force 𝐹SP Plate boundaries - Convergent (subduction) • The geometry of subducting plates is quite variable, but typical subduction angles are ~45° • If gravity drives subduction, why don’t slabs subduct with 90° dips? Fig. 1.9, Turcotte and Schubert, 2014 7 Plate boundaries - Convergent (subduction) • The geometry of subducting plates is quite variable, but typical subduction angles are ~45° • If gravity drives subduction, why don’t slabs subduct with 90° dip angles? Fig. 1.9, Turcotte and Schubert, 2014 8 Induced mantle flow and back-arc spreading Mantle corner flow Slab roll-back Fig. 1.11, Turcotte and Schubert, 2014 • • Subduction induces flows in the mantle that may control the slab dip angle These flows may also produce enigmatic back-arc spreading. Why is there extension at a convergent plate margin? 9 Test your might • You can find a short quiz about this lecture at https://elomake.helsinki.fi/lomakkeet/63002/lomake.html • • Please take the quiz to help me know what you have learned Your answers are anonymous and will not count in your course grade 10